High power semiconductor laser diodes have become important components in the technology of optical communication, particularly because such laser diodes can be used for fiber pumping (amplification of optical signals) and other high power applications. In most cases, long lifetimes (exceeding, for example, tens of thousands of hours), reliable and stable output, high output power, high electro-optic efficiency, and high beam quality are generally desirable.
Because modern crystal growth reactors are able to produce semiconductor material of a very high quality, the long-term reliability of high-power laser diode lasers has been found to strongly depend on the stability of the laser facets cleaved to form the opposing mirrors of the laser cavity.
Laser facet degradation is a complex physical and chemical reaction process that can be driven by light, current, and heat, and can lead to short-term power degradation during burn-in, long-term power degradation during normal operation, and, in severe cases, to catastrophic optical damage (COD) to the mirror surfaces themselves, resulting in complete failure of the devices. Complex oxides and point defects can form and be trapped at the interface between the reflective coating and the semiconductor material. As current is applied to the device, charge carriers diffuse toward the facet since the surface acts as a carrier sink (due to the presence of states within the band gap created by point defects and oxidation of the surface). Light emission from the diode can photo-excite charge carriers (electrons and holes) at the facet surface, which can electro-chemically drive an oxidation reaction at the facet. Additionally, electrons and holes generated by the absorbed light can recombine in a non-radiative manner, causing excessive heat development and contributing to the formation of lattice defects (both point defects and dislocations). Heating of the semiconductor material can induce thermal oxidation at the facet, further increasing the absorbing oxide layer thickness formed at the semiconductor-oxide interface. Excessive heat in such close proximity to the facet affects the electronic structure of the materials adjacent to the facet. Heat-induced shrinkage of the optical band gap of the semiconductor crystal increases absorption of light. More absorption leads to more heating and, consequently, a thermal runaway process is initiated which results in fast degradation of the facet, as well as the material adjacent to the facet and, ultimately, to COD and failure of the facet.
For many years, a process developed by IBM and referred to as “E2 passivation” has been used to address these concerns and minimize the possibility of COD. The E2 process involves the deposition of an amorphous silicon (a-Si) layer as a passivation coating over the cleaved facets. The essence of the E2 process is to chemically stabilize the chip facets by forming the silicon directly on the bare facet surface. While silicon is clearly an excellent choice to block/eliminate facet corrosion, it has the drawback that it absorbs light emitted from/by the laser diode. The absorbed light generates charge carriers which recombine non-radiatively, producing excessive heat and contributing to defect formation. These processes accelerate degradation of the facet and can initiate a thermal runaway situation leading to COD. Thus, the passivation layer cannot be too thick, since it would then absorb too much light, create too much heat and increase the probability of COD (it is known that the COD level sharply decreases with increasing thickness of the Si passivation film). The thickness limits are dependent on the wavelength, where shorter wavelengths are absorbed more strongly and the device performance situation becomes more critical for short wavelength laser diodes. However, while the passivation film should be as thin as possible in terms of light absorption, long-term life tests have shown that a film that is too thin does not sufficiently protect the surface of the facet. The thickness of the Si film is also a critical parameter for facets exposed to ions/atoms having higher energy, e.g., during the deposition of standard mirror coatings by ion beam sputtering, used to obtain the required mirror reflectivity.
From the above discussion, it is clear that one challenge to overcome is to reduce the absorption of light in the passivation film while at the same time keeping the passivation film sufficiently thick to protect the facet—which are obviously contradictory requirements in terms of the preferred thickness of the silicon passivation film. Additionally, the passivation structure needs to be formed in a manner that minimizes the number of charge carriers (produced by absorbed light) that are capable of reaching the chip cleaved facet by diffusion. Further, the passivation structure must not react with the facet, instead stabilizing the facet surface and preventing diffusion/migration of mobile atomic species/impurities to the facet. Moreover, the passivation structure itself should not be a source of facet contamination.